against the ten-times fatal dose. Each cubic centimeter
of this stronger serum was described as an antitoxic unit,
and, of course, contained ten immunizing units. Still
later it was shown that the limits of strength were by
no means reached, and he succeeded in making serums
three hundred times the normal strength, each cubic
centimeter of which contained 300 immunizing units,
or 30 antitoxic units.
In the course of the development of strength in the
serum the exact meaning of " immunizing unit" grad-
ually became obscured, until it is at present an expres-
sion of strength rather than one of quantity.
While it is difficult to define an immunizing unit, it is
not at all difficult for one skilled in laboratory technique
to determine the number present in a sample of serum.
There are three rules of practice:
1. Determine accurately the least certainly fatal dose
of a sterile diphtheria toxin for a standard guinea-pig.
2. Determine accurately the least quantity of the
serum that will protect a guinea-pig against ten times
the determined least certainly fatal dose of toxin.
3. Express the required dose of antitoxic serum as a
fraction of a cubic centimeter and multiply it by ten.
There will then be as many immunizing units in i c.cm.
of the serum as there are parts in the resulting fraction.
Example: It is found that o.oi c.cm. of toxin kills at
least 9 out of 10 guinea-pigs. It is then regarded as the
least certainly fatal dose. Guinea-pigs receive ten
times this dose (o.i c.cm.) and varying quantities of the
serum, measured by dilution, say, y^nr c.cm., -j^Vir c.cm.,
-g-fnnr c-cm- The first two live. The fraction -5-5^ is
now multiplied by 10; -^ir X 10 = ^J^, and we find that
there are as many units per cubic centimeter in the serum
as there are parts in the result—i. e., 250.
The most accurate definition of an immunizing unit is:
ten times the least amount of antitoxic serum that will
protect a standard (3OO-gram) guinea-pig against ten
times the least certainly fatal dose of diphtheria toxin.